Characterization of hot air drying and microwave vacuum drying of fingerroot (Boesenbergia pandurata)

Fingerroot (Boesenbergia pandurata) was subjected to hot air drying and microwave vacuum drying. Effective moisture diffusion coefficient during the hot air drying at 60 and 70 °C were 0.2073 × 10^?10 and 0.4106 × 10^?10 m² s^?1 respectively. By using the microwave vacuum drying (13.3 kPa) at the power of 2880 and 3360 W, the effective moisture diffusion coefficient were increased to 5.7910 × 10^?10 and 6.8767 × 10^?10 m² s^?1 respectively. Based on Lewis model, drying rate constants were 0.0002, 0.0004, 0.0061 and 0.0072 s^?1 for the hot air drying at 60 and 70 °C and the microwave vacuum drying at 2880 and 3360 W respectively. Compared with the hot air drying, the microwave vacuum drying decreased drying time by 90%. Rehydration ability of the microwave vacuum dried samples was also significantly improved (P ≤ 0.05), because of porous structure. In addition, the rehydrating water of the microwave vacuum dried samples contained higher b*‐value (yellowness) than that of the hot‐air‐dried samples (P ≤ 0.05).     

Experimental study on drying of pear slices in a convective dryer

The experiments were conducted on pear slices with thickness of 5 mm at temperatures of 50, 57, 64 and 71 °C with an air velocity of 2.0 m s^?1. Prior to drying, pear slices were pretreated with citric acid solution (0.5% w/w, 1 min, 20 °C) or blanched in hot water (1 min, 85 °C). Also, the untreated samples were dried as control. The shortest drying time of pear slices was obtained with pretreatment with citric acid solution. It was observed that whole drying process of pear slices took place in a falling rate period. Four mathematical models were tested to fit drying data of pear slices. According to the statistical criteria (R², χ² and RMSE), the Midilli et al. model was found to be the best model to describe the drying behaviour of pear slices. The effective diffusivity of moisture transfer during drying process varied between 8.56 × 10^?11 and 2.25 × 10^?10 m² s^?1, while the activation energy of moisture diffusion in pear slices was found to be 34.95–41.00 kJ mol^?1.     

Drying kinetics and thermal degradation of phenolic compounds and anthocyanins in pomegranate arils dried under vacuum conditions

The effects of temperature and pretreatment on drying kinetics and thermal degradation of phytonutrients present in pomegranate arils were investigated. The arils were divided into two groups, and half of the samples were pretreated by dipping into 80 °C hot water for 2 min. The drying process was conducted in the vacuum drier at the temperatures of 55, 65 and 75 °C. The fastest drying was completed at 75 °C after pretreatment of the samples. The highest anthocyanin–phenolic compound contents and antioxidant capacity were detected in the arils dried at 55 °C. Seven thin‐layer drying models were used to predict drying curves, and Arrhenius and Eyring–Polanyi models were employed to predict phytonutrient degradation kinetics. Activation energy for drying was 24.26 kJ mol^?1 for pretreated samples and 31.54 kJ mol^?1 for untreated samples. Effective moisture diffusivities were ranged from 1.43 × 10^?9 to 6.03 × 10^?9 m² s^?1.     

Thin-layer drying characteristics and modelling of mint leaves undergoing microwave treatment

The effect of microwave drying technique on moisture content, moisture ratio, drying rate, drying time and effective moisture diffusivity of mint leaves (Mentha spicata L.) were investigated. By increasing the microwave output powers (180–900 W) and the sample amounts (25–100 g), the drying time decreased from 12.50 to 3.0 min and increased from 6.60 to 16 min, respectively. To determine the kinetic parameters, the drying data were fitted to various models based on the ratios of the differences between the initial and final moisture contents and equilibrium moisture content versus drying time. Among of the models proposed, the semi-empirical Midilli et al. model gave a better fit for all drying conditions applied. By increasing the microwave output powers and decreasing the sample amounts, the effective moisture diffusivity values ranged from 3.982 × 10⁻¹¹ to 2.073 × 10⁻¹⁰ m² s⁻¹ and from 9.253 × 10⁻¹¹ to 3.162 × 10⁻¹¹ m² s⁻¹, respectively. The activation energy was calculated using an exponential expression based on Arrhenius equation. The relationship between the drying rate constant and effective moisture diffusivity was also estimated; and gave a linear relationship.     

Microwave–hot air drying applied to selected cassava cultivars: drying kinetics and sensory acceptance

The effects of three cassava cultivars produced in Brazil were assessed (IAC Espeto, IAC 14 and IAC Caapora) in order to obtain chips by precooking and hot air–microwave drying (70 °C, 95 W). The difference in the composition of cultivars has not influenced the drying of cassava slices. The drying kinetics presented a constant drying rate period followed by a decreasing drying rate with effective diffusion coefficient of 2.8–3.0 × 10^?10 m²s^?1. The hedonic means of all attributes ranged from ‘neither like nor dislike’ to ‘like moderately’. Therefore, the choice of the more suitable cultivar can be based on plant productivity and product yield. The IAC 14 cultivar resulted in the highest root production per plant, product mass per roots and chips yield.     

Influence of ultrasound pretreatments on diffusion coefficients,texture and colour of osmodehydrated sweet potato (Ipomea batatas)

The moisture and solute diffusivities, texture and colour of sweet potato pretreated in distilled water with ultrasound (UD), osmotic dehydration without ultrasound (OD) and osmotic dehydration with ultrasound (UOD) were investigated to determine the effects of ultrasound on osmodehydrated sweet potato. The moisture and solute diffusivities of all the pretreatments ranged from 2.89 to 4.95 × 10^?10 and 1.17 to 1.67 × 10^?10 m² s^?1, respectively. The moisture diffusivity of samples treated in UOD was significantly (P < 0.05) higher than that of OD and UD by 30.82% and 41.62%, respectively. Compared to the fresh sample, the firmness of samples treated in OD and UOD was reduced by 83.53% and 92.47%, respectively. UD increased the brightness (L*) and reduced the redness (a*) of sweet potato, while UOD enhanced the yellowness (b*) and chroma (C*) of sweet potato. The sweet potato samples treated in UOD had the highest total colour difference (ΔE).     

Mass transfer characteristics during convective,microwave and combined microwave–convective drying of lemon slices

BACKGROUND: The investigation of drying kinetics and mass transfer phenomena is important for selecting optimum operating conditions, and obtaining a high quality dried product. Two analytical models, conventional solution of the diffusion equation and the Dincer and Dost model, were used to investigate mass transfer characteristics during combined microwave‐convective drying of lemon slices. Air temperatures of 50, 55 and 60 °C, and specific microwave powers of 0.97 and 2.04 W g^?1 were the process variables. RESULTS: Kinetics curves for drying indicated one constant rate period followed by one falling rate period in convective and microwave drying methods, and only one falling rate period with the exception of a very short accelerating period at the beginning of microwave‐convective treatments. Applying the conventional method, the effective moisture diffusivity varied from 2.4 × 10^?11 to 1.2 × 10^?9 m² s^?1. The Biot number, the moisture transfer coefficient, and the moisture diffusivity, respectively in the ranges of 0.2 to 3.0 (indicating simultaneous internal and external mass transfer control), 3.7 × 10^?8 to 4.3 × 10^?6 m s^?1, and 2.2 × 10^?10 to 4.2 × 10^?9 m² s^?1 were also determined using the Dincer and Dost model. CONCLUSIONS: The higher degree of prediction accuracy was achieved by using the Dincer and Dost model for all treatments. Therefore, this model could be applied as an effective tool for predicting mass transfer characteristics during the drying of lemon slices. © 2012 Society of Chemical Industry     

Drying kinetics and effective moisture diffusivity of bamboo shoot slices undergoing microwave drying

Effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective moisture diffusivity (D_eff) of bamboo shoot was investigated using microwave drying. To study the effect of microwave power on drying, bamboo shoot samples (250 g) were dried at different power levels ranging from 140 to 350 W. To determine the kinetic parameters, drying data were fitted to various models based on the ratios of differences between initial and final moisture contents and equilibrium moisture content. Among the models proposed, Wang and Singh model gave a better fit for all drying conditions used. By increasing microwave output power, the D_eff values increased from 4.153 × 10^?10 to 22.835 × 10^?10 m² s^?1. A third‐order polynomial relationship was found to correlate the D_eff with moisture content. Further scope of this research work would include the effect of certain factors (shrinkage, case hardening, distortion of product and shape of bamboo shoot samples as an infinite slab) of practical significance to improve the model.     

Thin-layer drying characteristics of kurut, a Turkish dried dairy by-product

Kurut, which is made in villages of Eastern part of Turkey, is a sun‐dried dairy product. Thin‐layer drying behaviour of kurut at a temperature range of 35–60 °C, with 5 °C increments, and constant thickness of 0.5 cm and drying air velocity of 1.5 m s^?1 was determined in a convective type dryer. The data of sample weight, dry and wet‐bulb temperatures were recorded continuously during each experiment and drying curves obtained. The drying process took place in the falling rate period. Drying curves were then fitted to eleven mathematical models available in the literature to estimate a suitable model for drying of kurut. Two‐term model gave better predictions than other models and satisfactorily described the thin‐layer characteristics of kurut. The effective diffusivity varied from 2.444 × 10^?9 to 3.597 × 10^?9 m² s^?1 over the temperature range. The temperature dependence of diffusivity coefficient was described by the Arrhenius‐type relationship. The activation energy for moisture diffusivity was found to be 19.88 kJ mol^?1.     

Effect of microwave vacuum drying and hot air drying on the physicochemical properties of durian flour

Unripe durian cv. Monthong was subjected to microwave vacuum drying (MVD) at 1200, 1600 and 2200 W and hot air drying (HAD) at 40, 50 and 60 °C to produce durian flour. Drying rate of MVD and HAD was 0.34–0.58 kg water kg dry solid^?1 min^?1 and 0.02–0.06 kg water kg dry solid^?1 min^?1, respectively. An increase in drying rate by either increasing hot air temperature or increasing microwave power decreased the degree of crystallinity from 21.95% to 2.31% and from 7.72% to 4.05%, respectively. Moreover, the increased drying rate caused a decrease in endothermic enthalpy (ΔH_gel) and pasting properties. Starch content of the durian flour was 41.40–47.03%. The starch granule morphology of durian flour was disrupted which indicated gelatinisation of flour during drying. Due to a short drying process, the MVD flour had less a*‐value (P ≤ 0.05) than the HAD flour.     

Effect of air temperature,slice thickness and pretreatment on drying and rehydration of tomato

The effect of air temperature, pretreatment with alkaline emulsion of ethyl oleate (AEEO) and slice thickness on drying and rehydration characteristics of tomato slices was studied. Drying time decreased with pretreatment, but it increased considerably with the increase in air temperature and slice thickness of tomato. Besides, pretreatment was found to improve the rehydration ratio of tomato slices. The experimental drying curves obtained show only falling rate period. To estimate and select the suitable form of drying curves, five different mathematical models were applied to the experimental data. Among the mathematical models investigated, the Midilli et al. and logarithmic models satisfactorily described the drying characteristics of tomato slices with highest R² and lowest χ² and root mean square error. The effective moisture diffusivity varied from 3.123 to 10.03 × 10^?11 m² s^?1 over the temperature range studied, and the activation energy values varied from 59.6 to 70.2 kJ mol^?1.     

Effect of agitated bed drying on the retention of phenolic compounds,anthocyanins and antioxidant activity of roselle (Hibiscus sabdariffa L.)

An agitated fluidised bed dryer was employed to dry lixiviated roselle (Hibiscus sabdariffa L.) calyxes. The effects of operational drying variables on the chemical attributes such as the retention of monomeric anthocyanins (MA), phenolic compounds, polymeric colour, antioxidant activity and drying rate were analysed. Drying curves showed only decreasing falling rate period. To determine the kinetic parameters, the drying data were fitted to a Page's model, based on the moisture content ratio vs. the drying time; the k values ranged from 1.58 × 10^?5 to 6.28 × 10^?2 s^?1. The different drying conditions showed significant differences in the chemical parameters. When processing wet feeds, as lixiviated roselle calyxes, the mechanical assisted fluidisation can be considered as an innovative concept.     

Moisture transfer and microstructure change of banana slices during contact ultrasound strengthened far-infrared radiation drying

To explore water transfer and structure change during contact ultrasound strengthened far-infrared radiation drying (CU-FIRD) on banana slices, the effects of far-infrared radiation (FIR) temperature and contact ultrasound (CU) power on drying process, water transfer change, microstructure and pore evolution were studied. The results showed that the drying time could be shortened obviously by increasing FIR temperature and CU power. The effective moisture diffusivity values of banana slices were between 2.28 × 10⁻¹⁰–6.12 × 10⁻¹⁰ m²·s⁻¹, which increased with the rises of FIR temperature and CU power. The moisture changes in banana slices during CU-FIRD were analyzed by low-field nuclear magnetic resonance (LF-NMR). There were three kinds of water status in banana slices including bound water, immobilized water and free water. Free water was the main water in banana, and the increases of both FIR temperature and CU power could obviously shorten the removal time of free water. The immobilized water in banana slices increased first and then decreased during the drying process, and the improvements of FIR temperature and CU power could accelerate its mobility and migration. There was no significant change for bound water, which was the main water left in banana slices after drying. The SEM and shrinkage ratio results showed that higher FIR temperature and CU power could reduce the structure shrinkage of banana samples, increase the porosity, and generate more diffusion micro-channels with larger size, so as to speed up the migration process of water in banana. CU-FIRD could improve the porous structure of banana samples, enhance the heat and mass transfer process, and accelerate the migration and diffusion of water.     

Mass transfer during the osmotic dehydration of West Indian cherry

The main purpose of this work was to study water loss, solids gain, and weight and moisture reduction during the osmotic dehydration process of the West Indian cherry (Malpighia punicifolia). The diffusion coefficient of West Indian cherry was estimated by the inverse method using average moisture contents. Osmotic dehydration was examined for 12 h in a 65°Brix solution at temperature of 27 °C, without agitation, using a fruit:solution mass ratio of 1:4, 1:10, and 1:15. The kinetics and internal changes occurring during the osmotic dehydration of West Indian cherry are reported. The product’s drying kinetics was simulated using the diffusion model, and two optimization methods, Levenberg–Marquardt and Differential Evolution algorithm, were used to predict the diffusion coefficient. The results indicated that the two optimization methods performed similarly in estimating the diffusion coefficient adequately. The average calculated diffusion coefficient was 1.663 × 10^?10 m²s^?1, which is consistent with values reported in the literature.     

Ethanol pre-treatment to ultrasound-assisted convective drying of apple

For the first time, the ethanol as pre-treatment to the ultrasound-assisted convective drying of food was evaluated. Pre-treatments were performed by immersion of apple slices in ethanol (0–30 min). Pre-treated samples were convectively dried (50 °C, 1 m s⁻¹), without/with ultrasound (21.77 kHz, 20.5 kW/m³). As results, if both technologies were considered, conventional drying time reduction reached 70 ± 2%. From drying kinetics modelling, it was identified that ethanol pre-treatments mainly reduced the external resistance to mass transfer, while ultrasound had a greater influence on the internal one. In dried samples, as the ethanol pre-treatment time increased, the shrinkage decreased, and their rehydration capacity was greater. After rehydration, samples showed a decrease of >85% in viscoelastic characteristics. The antioxidant capacity and total phenolic content were better retained with ultrasound application. The obtained results corroborate that the proposed technologies are complementary significantly accelerating the drying without negative effects on physical properties.     

Effect of osmotic dehydration on dielectric properties,microwave vacuum drying kinetics and quality of mangosteen

Mangosteen (Garcinia mangostana Linn.) with and without osmotic dehydration (OD) in sucrose solution was dried by microwave vacuum drying at 1200, 1440 and 1680 W. Because of water loss (49.12–49.98 g 100 g^?1) and solid gain (9.31–11.62 g 100 g^?1) during OD, dielectric constant, loss factor and loss tangent of mangosteen were significantly increased (P ≤ 0.05) to 24.82–25.12, 11.52–14.18 and 0.47–0.50, respectively. With the decreased initial moisture content and the modified dielectric properties, drying time of osmotically dehydrated mangosteen was shorter than that of mangosteen without OD. Moreover, an increase in microwave power enhanced drying kinetics. With OD, Tg of dried mangosteen was increased from ?7.01, ?3.00 to 11.11–25.96 °C. Hardness and lightness (seedless part) were significantly increased (P ≤ 0.05). Structure of dried seedless mangosteen was well protected, resulting in the improved rehydration ability (P ≤ 0.05). Nonetheless, rehydration ability of the mangosteen containing seed was not improved (P > 0.05).     

Moisture diffusivity and shrinkage of broad beans during bulk drying in an inert medium fluidized bed dryer assisted by dielectric heating

Drying behavior of broad beans (Vicia faba) was studied in a pilot scaled fluidized bed dryer with inert particles assisted by dielectric heating. The effective diffusion coefficient of moisture transfer was determined by Fickian method at four different air drying temperatures of 35, 45, 55 and 65 °C. Correlations for moisture diffusivity as a function of moisture content and temperature of the drying medium were developed. The values of moisture diffusivity were obtained within the range of 1.27 × 10^?9–6.48 × 10^?9 m²/s and the activation energies for FBD and FBD + DE were found to be 27.71 and 17.10 kJ/mol, respectively. The shrinkage behavior of the broad beans was also investigated by considering the volume ratio of (V/V_O) to be function of moisture content alone and fitting a polynomial of the third order. The dielectric heating power was also found to be effective on the rate of drying.     

Combination of hot air drying and deep‐fat frying to reduce the oil content in chicken nuggets

The combination of hot air drying and frying to reduce oil uptake in chicken nuggets was analysed. Two air velocities (2 and 4 m s^?1) and two dry bulb temperatures (44 and 61 °C) were employed in the drying process, as a treatment before or after deep‐fat frying at 160 °C for 90 s in fresh soybean oil. The lowest fat content was obtained by frying followed of drying at 61 °C with air velocity of 2 m s^?1. When drying was applied before frying, mass transfer (moisture loss) during the drying process was modelled according to Fick’s second law; diffusion coefficients ranged between 1.03 and 3.33 × 10^?6 m² s^?1. When drying was applied after frying, a constant rate period was observed during drying process, with velocities between 0.002 and 0.02 kg_water/kg_{dry solids}·min. Scanning electron microscopy (SEM) allowed to observe differences in the topography of chicken nuggets obtained from frying or the combination of frying and drying.     

The effect of osmosis pretreatment on hot‐air drying and microwave drying characteristics of chili (Capsicum annuum L.) flesh

Chili flesh pretreated with or without osmotic dehydration (OD) was dried in the hot‐air drying (AD) oven at 50–80 °C or in the microwave drying (MD) oven at 60–180 W. Results showed that the samples osmotically treated in mixed solution (10% salt + 50% sucrose) had the best dehydration effect as compared with single salt or sugar solutions. During the drying process, osmotically treated samples had one falling‐rate period and their effective moisture diffusivities (D_eff) showed a rapidly linear increase with the decrease in moisture content, while directly drying samples showed a three‐phase falling‐rate period and their D_eff increased gradually at the initial period and then rapidly at the final period. When the moisture content decreased, the activation energy increased gradually; however, for AD after OD, it decreased. Among all the processes, MD at 60 W after OD presented the largest vitamin C retention rate and the best colour difference, needing less drying time.     

Original article: Characterisation of water species revealed in the drying operation of Todarodes pacificus Steenstrup using water proton NMR analysis

For the discrimination of water molecules during the squid‐drying process, the water distribution was characterised by water proton NMR and moisture diffusivity (De) analysis methods as a function of the water content (W₀). The proton NMR spectrum showed three peaks indicating three different species (species‐A, ‐B, and ‐C) distributed in the squid muscle, each of which had a characteristic behaviour of the relaxation time (T₂) as a function of the W₀. The 1/T₂ of species‐A was drastically varied at W₀ = 120%‐d.b., indicating two further categories, i.e., species‐A₁ and ‐A₂. Species‐A₁ is available at W₀ > 120%‐d.b. and was characterised as having De = 5.1 × 10^?10 m² s^?1, activation energy of moisture diffusivity (E_D) = 17 kJ mol^?1, and relaxation rate 1/T₂ = 74 s^?1, as evaluated by the proton NMR spectrum without depending on W₀. Species‐A₂ is available at W₀ < 120%‐d.b., indicating a distribution of De = 4.8 × 10^?10–1.7 × 10^?10 m² s^?1, E_D = 25–35 kJ mol^?1 and 1/T₂ = 1.8 × 10³–1.5 × 10² s^?1 with increasing W₀. Species‐A₁ and ‐A₂ were assigned as weakly restricted water and strongly restricted water, respectively.